The concurrent epidemics of obesity and inactivity are major public health concerns reported to account for about 400,000 deaths in the U.S. in 2000. To better promote physical activity and understand its role in the energy balance equation, this project addresses the broadly recognized need for improved instrumentation to monitor physical activity. Currently available technologies do not adequately support the epidemiological, clinical, or motivational efforts that are needed to effectively address this problem. A combined HR/motion sensing monitor has been recognized as a way to advance the state of the art in physical activity assessment. An innovative approach to the use of HR and motion data is proposed that obtains an estimate of the HR-EE relationship that is unique to each user and then applies that relationship to obtain EE from HR when motion data is unreliable or inappropriate. The specific aims of this project are 1) Develop and calibrate a dual-sensor prototype hardware platform; 2) Determine the feasibility of obtaining EE estimates for a set of physical activity modalities; and 3) Conduct a field evaluation under free-living conditions. The overall goal of this multi-phase SBIR project is to develop, validate, and commercialize a new tool for monitoring EE for broad-scale use in epidemiological research, in clinical settings, and by the general public as a method of activity motivation and monitoring. The anticipated outcome is a major advance in the state of the art in monitoring and assessment tools for addressing the tandem epidemics of obesity and inactivity. The final product we envision will be a high-performance, portable, low-cost physical activity monitor that employs both HR and motion to obtain a more accurate estimate of EE than possible using HR or motion sensors separately. The product will appear identical to the familiar HR monitor, with HR and motion sensors collocated in the chest strap communicating wirelessly with a watch display. We expect the price point to be comparable to that being charged for either HR monitors or accelerometer-based devices.